Metallic materials are used as elements, alloys or as composites in various mechanical devices, chemical devices, etc., depending on their physical and chemical properties. When they are used as parts which need to be corrosion resistant, only the surface of such parts needs to have sufficient corrosion resistance. It has been the practice, therefore, to coat the surface of a metal substrate with a material having superior corrosion resistance.
For example, it is known that titanium exhibits excellent corrosion resistance by forming a passive oxide film on the surface thereof. Thus, titanium has recently gained acceptance as a material for various machines, appliances and instruments such as chemical devices. In particular, in electrolysis apparatuses for sea water, brine, etc., pure titanium has been used widely as a material for an electrolytic cell or a substrate of an insoluble metallic electrode. However, since titanium is expensive, development of a method which permits a less expensive metal substrate to be covered with a thin titanium layer has long been desired. As such, however, crevice corrosion, etc., still tends to occur with pure titanium. The corrosion resistance of pure titanium is still not sufficient when titanium is used as an electrode substrate in electrolysis of strongly acidic electrolytic solutions containing hydrochloric acid, sulfuric acid, etc.
Attempts have, therefore, been made to coat the surface of the titanium with a platinum-group metal, such as palladium, or a platinum-group metal alloy, or with anticorrosive metals such as tantalum or niobium and alloys thereof.
Various methods to form a coating of an anticorrosive metal on the surface of a metal substrate have been proposed. For example, Japanese Patent Publication No. 415/1968 and Japanese Patent Application (OPI) No. 19672/1975 disclose a method for preventing crevice corrosion by bonding a titanium-palladium alloy material to a titanium substrate by welding or the like. Bonding by welding, however, requires a high level of welding skill. It is difficult, therefore, to apply this method to materials with a complex profile, and the strength of adhesion of such a material to the substrate is not entirely satisfactory.
On the other hand, various methods are known for depositing an anticorrosive material on the surface of a metal substrate by electroplating, chemical (electroless) plating, thermal decomposition, spraying, powder calcination, vacuum decomposition, etc., to coat the surface with such a material, and heat-treating the coated substrate (see, for example, Japanese Patent Publication Nos. 12882/1971, 2669/1973 and 24136/1973, and Japanese Patent Application (OPI) Nos. 25641/1973, 143733/1975, 4736/1978 and 18433/1978).
According to these methods, the thickness of the coating can be made as thin as is required. However, formation of micropores in the coated layer cannot be avoided, and heat-treatment must be performed in a vacuum, etc., for a long period of time. Because of these difficulties, prior art methods have not been able to provide products having a high degree of corrosion resistance and satisfactory adhesion of the coated layer to the substrate.